1. Field of the Invention
The present invention relates in general to disk storage systems and more particularly, to a method and apparatus for providing skew offset information for aligning the read element of a magneto-resistive (MR) head with the centerline of written data during a read operation.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is recorded on concentric tracks on either surface of one or more magnetic recording disks. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads. The read/write heads must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
To accurately write and read data, it is desirable to maintain the head on the center of the track. To assist in controlling the position of the head, each sector of the disk typically contains a number of servo bits accurately located relative to the centerline of the track. The signals produced by the servo bits are typically demodulated into position offset signals which are used to determine the position of the head relative to the track, and to move the actuator arm if the head is not located on the track centerline.
There has been developed a dual element transducer which includes a single write element and a separate read element which is constructed from a magneto-resistive material. Such dual element transducers are commonly referred to as magneto-resistive (MR) heads.
Because of manufacturing tolerances, the separate magneto-resistive read element may be off-center from the write element of the head. Therefore, if data is written off the center of the track, to read the data, the servo system must move the head slightly off-center so that the read element is centered with the written data.
Additionally, while the read and write elements may be aligned when the head is positioned over a particular track, when the head is moved to another track, the read element may no longer be aligned with the write element. For example, as shown in FIGS. 1A and 1B, when the head is positioned over a track along the outer diameter of the disk, the centers of the read and write elements (R and W respectively) are aligned. However, as shown in FIGS. 2A and 2B, when the head is positioned over a track along the inner diameter of the disk, the centers of the read R and write W elements are no longer aligned. The servo system must move the head slightly off-center, so that the read element R is centered with the written data. The routine of moving an MR head during a read operation is commonly referred to as micro-jogging.
It also has been determined that the skew offset information for an MR head located over the inner tracks of a disk is different from that of an MR head located over the outer tracks of a disk. In addition, such MR head skew offset information is typically nonlinear, and it also generally varies greatly between heads.
One approach in solving this problem is described in U.S. patent application Ser. No. 08/641,686 entitled "Method and Apparatus for providing Read and Write Skew Offset Information for a Magneto-Resistive Head," filed on May 1, 1996 and which is assigned to the assignee of the present invention. U.S. patent application Ser. No. 08/641,686 describes a method and apparatus for generating a skew or position offset signal for aligning the read element of an MR head with the centerline of written data, during a read operation. The disk has a plurality of tracks, one of which contains a calibration burst that allow the read element to be centered with the centerline of written data. This is accomplished by first writing the calibration burst on a predetermined number of sectors of the track. A profile of the calibration burst is obtained by micro-jogging the head from a first position that is -50% from the track centerline to a second position that is +50% from the track centerline, and sensing the magnitude of the calibration burst in various increments between the first and the second positions. For each microjogging position, the magnitude of the calibration burst is obtained. The microjogging position corresponding to the peak value of the calibration burst represents the offset between the read and the write elements. The offset signal is stored in memory and used in a servo routine to center the MR head during a read operation. The calibration burst may be included in a single track, a predetermined number of tracks, or on every track of the disk.
Through the implementation of the apparatus and method described in U.S. patent application No. 08/641,686 the read element of an MR head may be accurately aligned with the centerline of written data during a read operation. The microjogging process utilized in the technique described in U.S. patent Ser. No. 08/641,686 typically requires a minute. In establishing calibration bursts over four different sets of tracks on a disk, the technique described in U.S. patent application Ser. No. 08/641,686 may take up to 4 or 5 minutes.
Accordingly, there is a need in the technology for a method and apparatus for providing skew offset information used in the alignment of a magneto resistive head so that a read element of an MR head may be aligned with the centerline of written data during a read operation. It is desired that such alignment be provided in an efficient manner.